1. Field of the Invention
The present invention relates to a method that is used to observe a specimen in atomic force microscopy (AFM) and adapted for evaluation of physical properties of a specimen. The invention also relates to an atomic force microscope implementing this method.
2. Description of Related Art
Ultrasonic atomic force microscopy has been developed as a technique for evaluating the contact elasticity of the portion of a specimen surface contacted with a probe, from the resonance frequency of the flexural vibration of the cantilever of an atomic force microscope (AFM) operating in contact mode (K. Yamanaka and S. Nakano, Jpn. J Appl. Phys., Vol. 35 (1996), pages 3787–3792). This method has the feature that it can evaluate the contact elasticity of a stiff sample using a softer cantilever than is used in the presently commercially available contact elasticity evaluation technique of force modulation mode. This method is adapted for evaluation of metals, ceramics, semiconductors, and so on. Furthermore, the energy dissipation characteristics of a contact portion can be evaluated from the Q factor that is defined as the ratio of the resonance frequency to the resonant peak width (O. B. Wright and N. Nishiguchi, Appl. Phys. Lett., Vol. 71 (1997) pages 626–628). In addition, a method of evaluating elastic characteristics more completely by separating Young's modulus, shear modulus, and Poisson ratio by the use of torsional vibration of a cantilever has been proposed (K. Yamanaka and S. Nakano, Appl Phys. A., Vol. 66, (1998), pages S313–S317).
This ultrasonic atomic force microscopy is similar to recently widespread, frequency modulation mode, non-contact atomic force microscopy (NC-AFM) in that resonance of a cantilever is used. However, they have essential differences. In particular, in non-contact AFM, the cantilever vibrates at large amplitude of more than 10 nm and the probe moves away from the specimen. On the other hand, in ultrasonic AFM, the cantilever vibrates at a small amplitude of less than 1 nm, and the probe is kept in contact with the specimen. As a result, either the force gradient that is the ratio of displacement to force or contact elasticity is kept substantially constant over one period of vibration. The vibration can be analyzed by applying the linear elastic theory. Accurate quantitative evaluation is permitted. This is contrasted with non-contact AFM, which needs analysis of non-linear vibrations. Sometimes, complex chaotic behaviors participate in non-contact AFM. The ultrasonic AFM has the unique feature that structures and defects under the surface can be analyzed, as well as surface structures, because a strong contact force acts between the probe and the specimen. Therefore, this is expected as a new lattice defect analysis method that compensates for the drawbacks with electronic and mechanical material evaluation techniques in nanotechnology, as well as the drawbacks with electron microscopy. In practice, in international meetings about scanning probe microscopes and ultrasonic measurement related techniques, sessions on ultrasonic atomic force microscopes have been held since 1998. It is expected that ultrasonic AFM will spread considerably rapidly mainly to the field of materials.
Where the resonance frequency and the Q factor are measured by ultrasonic AFM, it is necessary that the frequency be swept and that a resonance spectrum be measured. Where resonance spectra are measured, if 10 spectra are time-averaged, it takes about 5 seconds to measure one spectrum even if a high-speed network analyzer is used. Therefore, visualization owing to mapping of resonance frequencies and Q factors takes a long time. Where an image of 256 pixels×256 pixels is created, 91 hours are necessary. The Q factor is an index indicating energy dissipation from a specimen surface, and includes information independent of resonance frequencies indicative of elastic characteristics. It is forecasted that material information will exist which cannot be discovered until resonance frequency and Q factor are both measured and compared. Consequently, there is a strong demand for a technique capable of measuring the distribution of Q factors and the distribution of resonance frequencies in a practical time and visualizing the distributions.